Polyolefin production using condensing mode in fluidized beds, with liquid phase enrichment and bed injection

ABSTRACT

The partially condensed fluid recycle stream of a fluidized bed polyolefin reactor operating in the condensing mode is split into two portions by a stream splitter. In a preferred mode, the smaller stream from the splitter contains a higher ratio of liquid to gas than the larger stream. One portion of the split stream is injected below the fluidized bed and the other, preferably with enhanced liquid content, is injected into the fluidized bed at a level above the product withdrawal level. Regulation of liquid injection above the product withdrawal level, as a function of liquid in the product discharge tanks, reduces the liquid in the product discharge system, resulting in improved discharge cycle times and more efficient conservation of monomer and other materials which might otherwise be lost in the discharge process.

RELATED APPLICATION

This is a continuation-in-part of U.S. patent application Ser. No.09/514,440 filed Feb. 28, 2000 by the same inventors and having the sametitle.

TECHNICAL FIELD OF THE INVENTION

This invention relates to gas phase exothermic reactions to makeparticulate product in fluidized bed reactors. The invention isdescribed with respect to olefin polymerization but is not limited tothe production of polymeric products; rather, it may be practiced inconnection with any exothermic reaction which is carried out in a gasfluidized bed with external cooling. It relates particularly toimprovements in the condensing mode of operation in which a portion ofthe fluidizing gas or fluid is withdrawn from the reactor, cooled toremove the heat of reaction, partially condensed, and recycled back tothe fluidized bed reactor. In the present invention, the recycle iseffected by splitting the recycle stream in at least two streamsdirected to different areas of the reactor.

BACKGROUND OF THE INVENTION

The gas phase fluidized bed process for polymerization permits areduction in energy requirements and capital investment compared withmore conventional processes. However, a limiting factor is the rate atwhich heat can be removed from an exothermic reaction reactor is limitedby the need to prevent excessive entrainment of solids in the fluidizinggas stream as it exits for recycle from the reactor. Hence the amount offluid which can be circulated and cooled per unit of time to remove theexothermic heat of polymerization is limited. As polymer product isproduced and removed from the fluidized bed, reactants and catalystmaterial are continuously supplied either to the recycle stream ordirectly to the reaction zone of the fluidized bed.

The quantity of polymer exothermically produced in a given volume of thefluidized bed is related to the ability to remove heat from the reactionzone. Adequate heat removal is critical to maintain a uniformtemperature within the fluidized bed and also to avoid catalystdegradation and polymer agglomeration. The temperature in the reactionzone is controlled below the fusing temperature of the polymerparticles. The dew point of the recycle stream is the temperature atwhich liquid condensate begins to form in the recycle stream. By coolingthe recycle stream below the dew point temperature and then injectingthe two phase mixture thus formed into the reaction zone, the heat ofvaporization of liquid is available to consume a portion of theexothermic heat of polymerization. This process is known as “condensedmode” operation of a gas phase polymerization process. As disclosed byJ. M. Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790 and by M.L. DeChellis, et al. in U.S. Pat. No. 5,352,749, operation in “condensedmode” permits an increase in the space time yield of the reactionsystem—that is, an increase in the amount of polymer produced per unitof time in a given fluidized bed reactor volume.

Below the reaction zone of the fluidized bed is a gas distributor gridplate. Its function is to provide a uniform distribution of the recyclestream into the bottom of the bed. Below the gas distributor grid plateis located a bottom head mixing chamber where the recycle stream isreturned after being compressed and cooled. As disclosed by S. J. Rhee,et al., in U.S. Pat. No. 4,933,149, flow deflection devices can bedesigned and positioned within the bottom head mixing chamber, to avoidexcessive build up of entrained solids within the bottom head mixingchamber when operating without partial condensation of the recyclestream. When operating in “condensed mode”, a deflector geometry asdisclosed in the '149 patent may be used to avoid excessive liquidflooding or frothing in the bottom head mixing chamber. However, as thecondensing level is increased to further enhance heat removal and spacetime yield, excessive amounts of liquid can exist in the bottom headmixing chamber. This can lead to liquid pooling and instabilityproblems.

The fluidized bed discharge process described by Aronson in U.S. Pat.No. 4,621,952 is an intermittent semi-batch process involving thetransfer, by pressure differential, of solid and gas through multiplevessels. Being semi-batch in nature, the product removal capacity of agiven facility is constrained by the time duration of the stepsnecessary to complete the process. The Aronson discharge processincludes interconnecting conduits with valves between the vessels topermit gas venting and pressure equalization. The gas contains valuableraw materials for the fluidized bed reaction system. The gas may includeunreacted monomers and comonomers; inert materials are also common.Aronson discloses that the discharge process obtains the desiredtransfer of solid material while minimizing gas losses. Aronson doesnot, however, monitor liquid in the product discharge tanks or injectfluid to a point higher than product withdrawal.

As disclosed by Jenkins, et al., in U.S. Pat. No. 4,543,399 and byAronson in U.S. Pat. No. 4,621,952 the polymer product is intermittentlywithdrawn from the fluidized bed at an elevation above the gasdistributor grid plate. At increasing levels of partial condensation ofthe recycle stream the likelihood increases that undesirably high levelsof liquid phase may exist in lower portions of the fluidized bed.Unfortunately during a product discharge event liquid can be carried outof the reactor along with the granular polymer and gases. Because of thedepressurization which takes place during product discharge, the liquidexpands and vaporizes, which may cause temperature reduction andpressure elevation within the discharge equipment. This can reduce thefill efficiency of the discharge system, and the reduction in fillefficiency in turn reduces the production capacity by increasing thetime to depressurize, and increases the raw material usage of theprocess. Accordingly, it has been difficult to increase the liquidcontent in the recycle stream to enhance the efficiency of removing theheat of reaction.

In Chinh et al, in U.S. Pat. No. 5,804,677, the patentees assert theydescribe the separation of liquid from a recycle stream; the separated,collected liquid is injected into the fluidized bed above the gasdistributor plate. The present invention also injects recycle liquidabove the distribution plate, but applicants' liquid is handled as aliquid/gas mixture and as a more or less predetermined fraction of therecycle stream, as a slip stream, divided simply and directly in therecycle conduit. Because of the applicants' manner of separating, we areable to enhance the ratio of liquid to gas in the slip stream ascompared to the withdrawn recycle stream, and thus simply and directly,without additional or special equipment, improve-heat exchangeefficiency and enhance the space/time yield of the process. In addition,we are able to optimize the product discharge cycle by coordinatingliquid volume in the discharge tanks with the rate of injection ofliquid above the distributor plate.

SUMMARY OF THE INVENTION

Our invention comprises splitting the recycle stream, after compressionand cooling, into at least two streams. One of the streams is returnedto the distributor grid plate or similar device below or near the bottomof the bed and the other(s) are returned to the fluidized bed at one ormore points above the distribution grid plate. The stream is split by aconduit segment designed for the purpose, sometimes herein called asplitter.

Preferably, the recycle stream is divided into two streams, the smallerof which is 5 to 30 percent of the total recycle stream and contains anenrichment of the liquid portion as a function of the relative momentumsof the liquid and gas components of the recycle stream, impacting in thesplitter, the liquid droplet size, and the particular configuration ofthe splitter. The liquid content (percentage by weight) of the smallerstream is preferably enriched to a percentage 1.01 to 3.0 times, morepreferably 1.1 to 2.5 times that in the stream prior to separation. Thelarger of the separated streams, having a lower liquid concentration buta higher volume, is recycled to the bottom head mixing chamber of thereactor vessel and introduced into the reaction zone in a uniformfashion more or less conventionally through a gas distributor gridplate. The smaller stream or streams having an enriched liquid phase, is(or are) recycled into the reaction zone at an elevation above the gasdistributor plate. Because of the lower ratio of liquid to gas in thelarger stream as compared to the original cooled/condensed stream, onlya minimal disturbance of the fluidized bed is imparted. We are thus ableto inject higher quantities of recycled liquid into the bed withoutcausing difficulties in the product withdrawal system.

An attractive novel feature of our modified recycle technique is thatthe separation of the recycle stream may be conducted without usingmechanical equipment such as separators, hydrocyclones, demisters,scrubbers, entrainment collection devices, pumps, compressors oratomizers. Rather, by withdrawing the small two phase stream or streamsfrom the recycle piping line, by the use of an elbow, bend, tee, orother piping configuration, an enrichment occurs of the liquid contentin the small stream. This occurs without any moving parts or theapplication of energy. This enrichment is due to the difference inmomentum between the lower density vapor phase and the higher densityliquid phase. As a result of inertia, the liquid droplet trajectoriesdeviate from the streamline of the bulk vapor flow. The liquid phase mayexist in the form of droplets ranging in size from 50 to 2000 microns.By selection of a suitable piping system, the small stream or streams,which have been enriched in liquid content, may be re-injected into thereaction zone of the fluidized bed at a location above the gasdistributor grid plate, preferably above the product withdrawal level.In this manner a large quantity of the condensed liquid exiting thecooler can be injected into the upper portions of the fluidized bedwithout separating the gas and liquid phases using mechanical equipment.This is an advantage over the methods disclosed by Chinh, et al. in U.S.Pat. Nos. 5,541,270, 5,668,228, 5,733,510 and 5,804,677 (see the summaryabove) in that the financial costs for mechanical equipment such asseparators, hydrocyclones, demisters, scrubbers, entrainment collectiondevices, pumps, compressors or atomizers are not incurred. Some of thesehave moving parts and all entail substantial maintenance problems. Theadvantage compared to conventional technology incorporated by UnionCarbide Corporation and disclosed by Jenkins, et al. in the U.S. Pat.No. 4,543,399 and U.S. Pat. No. 4,588,790 is that the liquidre-injection into the reaction zone of the fluidized bed can occur atone or more points above the gas distributor grid plate withoutsubstantial disturbance of the fluidized bed.

We use the term “splitter” to mean an elbow, bend, tee, or other conduitsegment having an inlet (upstream) portion and two or more outlet(downstream) portions. The outlet portions may be configured, either bya reduction in overall internal diameter or by one or more obstructionsor diversions, to provide a resistance to the flow of fluid, which will,to at least some degree, cause liquid to coalesce or accumulate in atleast one of the exit portions. Preferably, the incoming fluid isdivided into a primary stream containing a high ratio of gas to liquidcompared to the secondary stream(s) and at least one secondary or slipstream containing a relatively high ratio of liquid to gas compared tothe fluid entering the splitter. The secondary stream may be larger—thatis, the pipe diameter for the secondary stream may be greater than thatof the primary stream, and/or the flow of total fluid may be greater inthe secondary stream, but we prefer that the secondary stream—the streamcontaining a higher ratio of liquid to gas—be of a smaller diameter thanthe primary stream. We use the terms “secondary stream,” “bypass,” and“slip stream” interchangeably. There may be more than one secondary orslip stream. Further, the primary stream may be directed to anothersplitter to be further split into additional streams, at least onehaving an enhanced ratio of liquid to gas compared to the fluid enteringit, for additional injection into elevated regions of the reactor,preferably above the product withdrawal level. However, fluidization ofthe bed 2 (FIG. 1) must be maintained throughout; fluidization requiresa sufficient quantity and velocity of fluid through line 3 todistribution plate 7.

We use the term “through a direct passage” to mean that the slip streamis passed directly from the elbow, bend, tee, or other conduit segment(splitter) to the reaction zone of the reactor, or to the upstream endof another splitter, without going through any mechanical equipment suchas separators, hydrocyclones, demisters, scrubbers, entrainmentcollection devices, pumps, compressors or atomizers.

The ability to pass the slip stream through a direct passage into thereaction zone of the fluidized bed is enhanced by the usual slightreduction in pressure in the fluidized bed from its lower region to itsupper region. Commonly, the pressure in the upper regions is from 0.04to 0.15 psi per foot of height less than that in the lower regions ofthe bed. Thus, the higher the injection point in the bed, the greaterwill be the difference between the pressure in the slip stream and thatin the reactor bed, which of course assists the flow of the secondarystream into the fluidized bed. Generally, we will inject the secondarystream at one or more points between six inches and 10 feet above thedistributor plate of a commercial polyolefin reactor such as that shownin FIG. 1, but we prefer to inject the secondary stream at a heightbetween eighteen (18) inches and ninety-six (96) inches above thedistributor plate. Recycle injection is preferably above the point ofproduct withdrawal.

Our invention includes a conduit segment which will provide a slipstream through a direct passage from a preferred elbow configurationdefining a settling chamber and a discharge duct for the slip streamlocated at the bend of the elbow. More particularly, our inventionincludes a splitter for splitting a partially condensed recycle streamfrom a recycle stream in a fluidized bed polyolefin reactor, thesplitter comprising an inlet portion, a primary outlet portioncommunicating with the curved portion, a secondary outlet portion, thesecondary outlet portion preferably including a settling chamber locateddownstream from, adjacent to, and on the outside radius of said curvedportion, and a slip stream conduit communicating with the settlingchamber, the slip stream conduit preferably having a smaller effectivediameter than that of the primary outlet portion and of the settlingchamber.

This invention is an improvement in the “condensed mode” of operation.As disclosed by Jenkins, et al. in U.S. Pat. Nos. 4,543,399 and4,588,790 and by DeChellis, et al. in U.S. Pat. No. 5,352,749, operationin “condensed mode” permits an increase in the space time yield of thereaction system—that is, the amount of polymer produced per unit of timein a given fluidized bed reactor volume. Also disclosed by DeChellis inthe aforementioned U.S. patent is that excessively high levels of liquidintroduced to the fluidized bed may promote the formation of undesirablepolymer agglomerates, the presence of which can lead to bed collapse andreactor shutdown. Excessive liquids can also influence local bedtemperatures which yield undesirable inconsistencies in polymer productproperties.

Our invention provides an increase in the space time yield (polymerproduction per unit of time) of a reaction system of a given volume,compared to other condensed mode techniques. In particular, theseparation of the partially condensed recycle stream is accomplishedwithout the use of costly mechanical separating devices such asseparators, hydrocyclones, demisters, scrubbers, entrainment collectiondevices, pumps, compressors or atomizers.

An important aspect of our invention is that the injection of liquidabove the point of product withdrawal reduces the quantity of liquidcarried out of the fluidized bed with the solid and gas during thedischarge process. This improves the product removal capacity and theraw material efficiency of the semi-batch discharge process. Regulationof the liquid split as a function of monitored or modeled liquid in thedischarge tanks enhances the efficiency of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the gas phase fluidized bedexothermic polymerization process, including the recycle streamseparation and liquid enrichment system.

FIG. 2 depicts an elbow installed at the desired separation point forthe recycle stream to provide a slip stream for insertion to thereaction zone of a fluidized bed such as in FIG. 1.

FIG. 3 is a further illustration of a preferred splitter to emphasizeits optional settling zone.

FIGS. 4, 6, 7, and 8 are velocity profiles with accompanying dropletdistribution data within the preferred elbow configuration, as predictedby a computer simulation for various conditions and products.

FIG. 5 is an 8-droplet projectory profile in a preferred elbowconfiguration.

FIGS. 9, 10, 11 and 11 a show variations in elbows which may be used inour invention.

FIG. 12 is a schematic illustration similar to FIG. 1, includingmultiple discharge tanks in parallel, similar to apparatus described byAronson in U.S. Pat. No. 4,621,952. The use of this apparatus in ourinvention is described in Examples 17, 18, 19 and elsewhere herein.

FIG. 13 illustrates a typical pressure equalization between vessels 10and 10′.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, a preferred embodiment of thesystem for carrying out the process of the invention is shown. Thereaction vessel 1 contains a fluidized bed 2 of particulate product witha fluid recycle stream entering the bottom inlet line 3 and exitingthrough line 4 from expanded section 28. Reaction catalyst is suppliedintermittently or continuously from a reservoir 5 to a point 6 above thegas distributor grid plate 7 immediately below the fluidized bed 2.Polymer product is withdrawn intermittently or continuously through thedischarge line 8 through valve 9 into tank 10. The discharge tank 10 maybe connected by valve 11 in series with another tank 12. The polymerproduct is subsequently transferred to downstream processing equipmentthrough valve 13 and line 14 with the optional addition of transferassist gas through line 15. There are other ways known in the art toaccomplish this product removal process and some are disclosed byAronson in U.S. Pat. No. 4,621,952. The invention is not limited to theproduction of polymer products, but rather may be practiced inconnection with any exothermic reaction which is carried out in a gasfluidized bed with external cooling. Variations are well known in theart for maintaining the bed in a fluidized state, feeding the catalyst,feeding the monomer(s), and recovering unreacted monomer from theproduct stream. Our invention can readily accommodate such variations asapplied by persons skilled in the art.

The recycle stream exiting through line 4 of our preferred system ispassed through a compressor 16 and cooler 17. As the reaction proceedsraw materials such as monomers are introduced possibly to the recyclestream 3 through line 18 or at other locations of the process such asdirectly into the fluidized bed 2 or into the recycle line 4. Themaximum velocity of the recycle stream exiting through line 4 isconstrained to avoid excessive entrainment of polymer particles into therecycle line. This is because the particles may tend to plug and foulcompressor 16, cooler 17 and gas distributor grid plate 7. The functionof the gas distributor grid plate is to provide a uniform distributionof the recycle stream into the bottom of the bed. The reaction vessel 1normally includes an expanded upper section 28 to permit a localdecrease in the gas velocity and reduce the propensity for the removalof polymer particles from the reactor through the top line 4. Theminimum velocity of the recycle stream in line 4 is normally severaltimes the minimum needed to suspend the particles within the bed 2 in afluidized state.

The cooler 17 removes the exothermic heat of polymerization and the heatof compression. Adequate heat removal is important to maintain a uniformtemperature within the fluidized bed and also to avoid catalystdegradation and polymer agglomeration. The temperature in the reactionzone is controlled below the fusing temperature of the polymerparticles. Under steady state conditions, the temperature of the bed isnearly uniform. A temperature gradient exists within a small section ofthe bottom layer of the bed. Typically (and particularly in the absenceof liquid introduction to the higher regions of the bed, as in thepresent invention) the temperature gradient does not extend above thefirst 12 inches of the bottom of the bed 2. The temperature gradient iscaused by the lower temperature of the recycle stream which is returnedto the bottom of the fluidized bed 2. As disclosed by Jenkins et al. inU.S. Pat. Nos. 4,543,399 and 4,588,790, the discharge temperature of therecycle stream exiting the cooler 17 may be below the dew pointtemperature of the mixture. The dew point is the temperature at whichthe mixture begins to condense. The recycle stream thus may be partiallycondensed as it exits the cooler 17. This is known as “condensed mode”of operation. The amount of gas and non-condensed vapor in the recyclestream and the velocity of that non-liquid phase should be sufficient tokeep the liquid portion of the recycle stream suspended, in order toavoid settling and accumulation of liquid in the recycle line 4. Asdisclosed by Rhee et al. in U.S. Pat. No. 4,933,149, a deflector device23 can be designed and positioned in the bottom of the reactor 1 topermit stable operation with or without partial condensing of therecycle stream in line 4.

In accordance with this invention the partially condensed recycle streamwhich exits the cooler 17 is separated into two or more streams. Theseparation is conducted in a segment 22 of the recycle conduit includingan elbow, bend, tee or any other splitter which will separate at leastone slip stream, with or without achieving liquid enrichment in the slipstream line 19. By varying the design and placement of the conduitsegment 22, the “slip stream” line 19 (sometimes herein called a bypassor a bypass line) can be enriched in liquid content relative to theprimary exit stream. In particular line 19 preferably contains 5 to 30percent of the material in the total recycle stream. By causing theseparation to occur at an elbow, bend, tee or other splitter, anenrichment occurs of the liquid content in the small or slip stream(bypass) in line 19. This is due to the difference in momentum betweenthe lower density vapor phase and the higher density liquid phase asthey strike the outside radius of the splitter (conduit segment 22), tobe explained further with respect to FIG. 2. The stream in line 19,which has preferably been enriched in liquid content, may be re-injectedinto the reaction zone of the fluidized bed at one or more locations 20above the gas distributor grid plate 7. In this manner a large quantityof the condensed liquid exiting the cooler can be injected directly intothe fluidized bed at levels above the distributor plate withoutseparating the gas and liquid phases using mechanical equipment. There-injection point of the stream in line 19 may be at multiple locationsaround the circumference of the fluidized bed and at multiple locationsalong the axis of the fluidized bed. The locations are chosen to insurerapid dispersion and vaporization of the relatively cool liquidcontained in the two-phase stream in line 19. Flow in line 19 may befurther controlled by valve 21. In particular, a preferred practice isto manipulate valve 21 to maintain a desired pressure differential of upto 10 psi, preferably 0.01 to 3 psi, between inlet location 20 and thepoint of entry into the reactor of line 3, the point lower in the bedhaving the higher pressure. Line 3 may extend into the reactor. In atypical large commercial fluidized bed polyolefin reactor, haying astraight section of perhaps 50 feet in height, the lowest re-injectionpoint will be at least 12 inches above the gas distribution plate 7.Re-injection should be in a zone of the fluidized bed where it willvaporize quickly, and usually this is in the lower half, preferably thelower third, of the bed. In addition, it is preferably injected abovethe product discharge level illustrated by product discharge line 8.However, in principle, the recycle fluid may be re-injected anywhere inthe fluidized bed. We prefer that it be at one or more points in thelower third of the bed, for example 8-10 feet above the gas distributionplate.

The relative liquid enrichment of the usually smaller two-phase streamin line 19 can be affected by the velocity of the bulk stream exitingthe cooler 17 and also by the density difference between the vapor andliquid phases. The velocity is determined partly by the physicaldimensions of the recycle line and the design and operatingcharacteristics of the compressor 16. As disclosed by Jenkins, et al.(see claims 21 and 22) in U.S. Pat. No. 4,588,790, the addition of inertcomponents can be used to adjust the dew point of the recycle streammixture. With respect to this invention, as the difference between thedensity of the condensed liquid phase and the density of the vapor phaseis increased, the enrichment of liquid in the preferably small secondaryrecycle stream in line 19 is increased. The densities of the liquid andvapor phases change as the composition of the recycle stream is changed.The densities may be manipulated by the addition or removal of morevolatile or less volatile (i.e. more dense and/or less dense) chemicalcomponents to the recycle stream or reactor. Thus the inerts added topromote condensation may be selected also to enhance the enrichment ofliquid into the secondary recycle stream in line 19 by assuring asignificant difference in densities. Also variations in the operatingconditions within the reactor 1, depending upon the catalyst type beingsupplied through line 6, and the raw materials being supplied throughline 18, will alter the density difference between the vapor and liquidphases exiting the cooler 17. The specific operating conditions aregenerally chosen and controlled to be nearly constant and uniform valuesto produce a consistent desired product which exits the system from line14.

A preferred elbow-type splitter (conduit segment 22) is illustratedfurther in FIG. 2. FIG. 2 is an enlargement of the conduit segment 22 ofFIG. 1, and shows a preferred configuration wherein partially condensedrecycle fluid enters from cooler 17 and is split into two streams, aprimary stream which enters line 3 and a secondary stream which proceedsthrough line 19. Line 19 is connected to the outside curve 24 of conduitsegment 22, downstream of a settling chamber 25. The particulardimensions and curvatures of the conduit segment 22 which affect itsefficiency as a fluid splitter may vary with product properties andprocess parameters as well as desired recycle rates. Generally, thesecondary exit line 19 will preferably have a diameter from 5% to 20% ofthat of primary exit line 3, and settling chamber 25 will have adiameter larger than that of line 19, from to 10% to 30% of primary exitline 3. The illustrated settling chamber 25 has a downstream wall 29 toprovide an obstacle to the free passage of liquid into line 19.

A variant of settling chamber 25 is shown in FIG. 3, which is furtherenlarged and contains a computer-generated illustration of theaccumulation of liquid in stagnant region 27 of settling chamber 26. Inthis version, downstream wall 29 is substantially vertical asdepicted—that is, in a plane transverse to the flow in line 4 (FIG. 1)coming from cooler 17—as contrasted with the version illustrated in FIG.2. Stagnant region 27 is shown with dotted lines; it represents theregion in which liquid accumulates for transport into line 19 and beyondto the fluidized bed as described with respect to FIG. 1.

The computer analysis depicted graphically in FIGS. 3 and 4 reveals thatthe liquid content of the collection stream is enriched. Results arepresented below in TABLE 4. Liquid collection efficiencies of 24.00 to28.82% were obtained for PE (polyethylene) operation, with a feedmixture containing 7.65 wt % liquid. A similar analysis was done for PP(polypropylene) operation with a feed mixture containing 15.6 and 24.9wt % condensed liquid. The collection efficiency of the liquid dropletsinto the bypass stream is relative to the total amount of liquid whichexits the cooler 17. A vapor rate through the bypass stream is alsocomputed. An enrichment of liquid into the bypass stream occurs when theweight percentage of liquid in the bypass stream exceeds that in themixture exiting the cooler 17. In the following tables, the “bypassstream enrichment ratio” therefore represents the amount of liquid inthe slip stream (bypass stream) compared to the amount leaving thecooler 17. PP liquid collection efficiencies of 20.30 to 24.20% wereobtained. These are shown in Table 5. The collection efficiency isstrongly correlated with droplet size. As shown in Tables 4 and 5,splitters can be designed to achieve liquid enrichments in the bypassstreams in excess of 1.1 times the liquid content of the stream enteringthe splitter.

Model and Geometry Assumptions

The FLUENT software program (Fluent, Inc., Lebanon, N.H.) computed vaporand liquid flow distribution within the piping and elbow region. TheSIMPLE (semi-implicit method for pressure-linked equations) algorithmwas employed along with the automatic grid generation features of theUnstructured Mesh Version of FLUENT.

A steady state model considered conservation of mass and momentum. Itemployed a standard k-e turbulence model with 5% inlet intensity. Thislower level of turbulence was used since the flow profile in the CycleGas piping upstream of the elbow may be assumed to be well defined andrelatively uniform. Table 1 includes results for a 3-dimensionalgeometry. An abrupt 90° elbow is illustrated in FIGS. 9 and 10. An ASMEclass B16.9 90° elbow with a rectangular bypass duct is represented inFIGS. 11 and 11 a. In FIGS. 9, 10, and 11, the inlet diameter is 40.25and the diameter of exit line 3 is also 40.25. In FIG. 11, note that thecenter of rectangular bypass line 53 is at the same level as the centerof inlet 4. TABLE 1 CYCLE PIPING AND BYPASS SLIP STREAM COLLECTIONDEVICE GEOMETRY Inlet Cycle Pipe Bypass Device Cylindrical DimensionsDiameter Reference Device Description (inches) (inches) FIGURES2-dimensional Elbow with Chamber 18.00 34.50 3, 4, 5, 6b, 7, 8 SettlingChamber with Nozzle 4.00 Collection Nozzle 3-dimensional Abrupt ElbowNozzle 12.00 40.25 9, 10 with Collection Nozzle 3-dimensional ASME B16.9Duct Width 18.0 40.25 11 Cylindrical Pipe and Elbow Duct Height 9.0 withRectangular Collection DuctDiscrete Liquid Phase Droplet Model

Liquid droplet particle tracking was done using the Lagrangian discretephase approach. The Lagrangian particle tracking feature allows for thecomputation of the trajectories of groups of individual droplets. Thisfeature accounts for droplet inertia, drag, buoyancy and gravity forces.The drops were injected across the inlet boundary at evenly spacedinjection points.

As shown in FIG. 4, defining zones along the wall and at exit boundariesallowed the monitoring of position for the droplets which were captured.All of the wall zones A, B, C, D, E, F, G, and H were assigned theproperty to capture any liquid droplets whose trajectory terminatedthere. Droplets in zones C, D, E, F, G, and L would be collected. Theliquid collection efficiency was defined by dividing the dropletquantity in these zones by the total injected. Droplets in contact withzone B, the bottom of the Cycle Gas pipe, could possibly be re-entrainedinto the mixture.

Graphical fluid phase profiles were used to qualitatively assess liquidcollection efficiency. A minimum mean droplet size of 104 microns(0.0041 inch) and a maximum droplet size of 312 microns (0.0123 inch)were used. The enrichment results are based on the average of thecalculations for the two droplet sizes. FLUENT post-processing usingparticle tracking was conducted using both the droplet sizes.

After solving for the vapor velocity, pressure and turbulence profilesfor the continuous phase, inert liquid droplets were injected at theinlet boundary. FIG. 5 illustrates a typical polyethylene droptrajectory for 8 drops of size 0.0041 inch (104 micron) with a velocityof 35 ft/sec. Droplet size and inertia were found to be stronglycorrelated with capture efficiency of the elbow device. FIGS. 4 and 6illustrate polyethylene process recycle velocity profiles with 0.0041inch and 0.0123 inch elbow inlets respectively, at different velocities.FIGS. 7 and 8 are similar drop trajectory results for a polypropyleneprocess.

FIG. 9 illustrates a right angle elbow having a bypass line 50 slightlybelow the level of inlet line 4, FIG. 10 shows a bypass line 52 slightlyabove inlet line 4, and FIG. 11 shows a rectangular bypass duct 53having a center at the same level as the center of inlet 4, as in ASMEmodel B16.9. FIG. 11 a shows the cross section of bypass duct 53, havingdimensions of 9 and 18 inches. Preferably, the bypass duct in any of ourconfigurations will have a hydraulic diameter of 5 to 30%, morepreferably 5 to 20%, of the diameter of the primary outlet. All of theseconfigurations are satisfactory in our invention.

Model Fluid Property Assumptions

For analyses, reactor fluid properties similar to those of commoncommercial homopolymers of ethylene and propylene were assumed. Theseare shown below in TABLE 2. The FLUENT continuous phase was representedby the Cycle Gas vapor properties. TABLE 2 CYCLE GAS FLUID PROPERTIESNEAR REACTOR INLET Polypropylene Polymer Polyethylene A B Pressure, psia374.4 556.2 501.2 Temp, deg C 53.0 63.1 57.2 Liquid wt % 7.65 15.6024.96 Density, lb/ft³ mixture 1.95 5.51 5.25 vapor 1.80 4.80 4.14 liquid38.6 26.4 27.30 Viscosity, cp vapor 0.016 0.014 0.013 liquid 0.180 0.0350.044

Model Boundary Conditions and Computational Model Results

Referring to FIG. 4, at steady state, a uniform velocity isothermalvapor enters the left side of the computational domain. Some fluid exitsthrough the chamber and collection nozzle (zone E and outlet L). Most ofthe fluid is turned at the elbow and travels vertically. It exits at thetop boundary (outlet zone K). A velocity specification and referencepressure was used at the inlet boundary (zone J). Pressure and/orvelocity specifications were employed at both of the exit boundaries(zones K and L).

The amount of vapor exiting through the collection nozzle (zone E andoutlet L) is a function of the pressure difference between the two exitboundaries (zones K and L). Outlet L is attached to recycle piping (slipstream line 19) to transport the mixture to an upper portion of thefluidized bed 2 (FIG. 1). The control of the flow bypassing the Reactorbottom head 23 and plate 7 may be made by varying the resistance in line19 and valve 21.

The liquid droplet recovery information is summarized below in Table 3The liquid droplet recovery was defined by dividing the droplet quantityin zones C, D, E, F, G, and L by the total injected. The vapor recoveryis the percent of total vapor flow which exits through the collectiondevice at the elbow. Since the liquid droplet recovery exceeds the vaporrecovery, a net enrichment of liquid occurs in the secondary streamexiting the collection nozzle. TABLE 3 LIQUID DROPLET RECOVERY FOR 1,000TRAJECTORIES - ELBOW WITH SETTLING CHAMBER AND BYPASS DUCT - FIG. 4Bypass Bypass Vapor Bypass Liquid Inlet Liquid Inlet Velocity RecoveryDroplet Recovery Polymer (wt. %) (feet/sec) (% of inlet) (microns) (% ofinlet) Polyethylene 7.65 35 19.59 104 24.00 Polyethylene 7.65 35 19.59312 27.90 Polyethylene 7.65 55 19.61 104 24.40 Polyethylene 7.65 5519.61 312 28.82 Polypropylene 15.60 25 17.38 104 20.30 Polypropylene15.60 25 17.38 312 22.30 Polypropylene 15.60 35 17.28 104 20.30Polypropylene 15.60 35 17.28 312 22.50 Polypropylene 24.96 25 18.45 10421.50 Polypropylene 24.96 25 18.45 312 23.70 Polypropylene 24.96 3518.44 104 21.70 Polypropylene 24.96 35 18.44 312 24.20

PE liquid droplet population distributions are illustrated in Table 4for polyethylene and Table 5 for polypropylene. FIG. 5 illustratestypical liquid droplet trajectory profiles for a subset case of onlyeight liquid droplets. TABLE 4 LIQUID DROPLET DISTRIBUTION FORPOLYETHYLENE CYCLE GAS Vapor Density 1.80 1.80 1.80 1.80 lb/ft³ LiquidDensity 38.60 38.60 38.60 38.60 lb/ft³ Condensed Liquid 7.65 7.65 7.657.65 weight percent Cycle Pipe Velocity 35.00 35.00 55.00 55.00 feet/secDroplet Mean Size 104.00 312.00 104.00 312.0 microns PressureDifferential 1.0 1.0 2.3 2.3 (Outlet - Bypass) (lb/in²) DropletDistribution by Zone (reference FIG. 4) (% of 1,000 trajectories) A 0.000.00 0.00 0.00 B 3.80 13.00 2.00 6.80 C 0.00 1.20 0.00 0.20 D 1.10 1.100.60 1.40 E 1.20 1.30 0.90 0.90 F 0.00 0.30 0.00 0.70 G 2.00 5.30 1.906.30 H 5.60 16.40 5.40 17.00 J 0.00 0.00 0.00 0.00 K 66.60 42.70 68.2046.90 L 19.70 18.70 21.00 19.80 Bypass percentage of Inlet Vapor 19.5919.59 19.61 19.61 Liquid 24.00 27.90 24.40 28.82 Bypass LiquidEnrichment 1.21 1.38 1.22 1.44 (weight ratio to Inlet)

TABLE 5 LIQUID DROPLET DISTRIBUTION FOR POLPROPYLENE CYCLE GAS - ELBOWWITH SETTLING CHAMBER AND BYPASS DUCT Vapor Density (lb/ft{circumflexover ( )}3) 4.80 4.80 4.80 4.80 4.14 4.14 4.14 4.14 Liquid Density(lb/ft{circumflex over ( )}3) 26.40 26.40 26.40 26.40 27.30 27.30 27.3027.30 Inlet Liquid (weight %) 15.60 15.60 15.60 15.60 24.96 24.96 24.9624.96 Inlet Velocity (ft/second) 25. 25. 35. 35. 25. 25. 35. 35. BypassLiquid 104. 312. 104. 312. 104. 312. 104. 312. Droplet Size (microns)Pressure Differential 0.9 0.9 1.7 1.7 0.9 0.9 1.7 1.7 (Outlet-Bypass)(lb/in{circumflex over ( )}2) Droplet Distribution by Zone (referenceFIG. 4) (% of 1,000 trajectories) A 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 B 3.00 9.90 1.90 6.20 3.50 11.20 2.20 7.10 C 0.00 1.10 0.00 0.400.00 1.30 0.00 0.70 D 1.10 1.20 0.60 1.60 1.30 1.30 0.80 1.40 E 0.901.00 1.60 1.10 0.90 0.90 1.40 1.10 F 0.00 0.10 0.00 0.20 0.00 0.00 0.000.30 G 1.20 1.70 1.30 1.90 1.30 1.90 1.40 2.10 H 2.90 6.80 2.70 6.703.30 7.80 3.00 7.70 J 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K 73.8061.00 75.10 64.40 71.70 57.30 73.10 61.00 L 17.10 17.20 16.80 17.5018.00 18.30 18.10 18.60 Bypass percentage of Inlet Vapor 17.38 17.3817.28 17.28 18.45 18.45 18.44 18.44 Liquid 20.30 22.30 20.30 22.50 21.5023.70 21.70 24.20 Bypass Liquid Enrichment 1.14 1.23 1.15 1.24 1.12 1.201.13 1.22 (weight ratio to Inlet)

This analysis reveals that it is feasible to collect about 20.30 to24.20% of the liquid in the recycle stream into the smaller splitstream, and to increase the ratio of liquid to gas in the smaller streamas compared to the ratio in the main recycle stream. Liquid collectionefficiency is strongly correlated with droplet size and density, and thedifference in momentum between the vapor and liquid phases.

Computational Model Conclusions

Two- and three-dimensional models were developed for the recycle fluidpiping elbow. Results were presented from the FLUENT Unstructured Meshsoftware program. Liquid droplet particle tracking was done using theLagrangian approach. Recycle fluid properties typical of commercialpolyethylene and homopolypropylene processes were assumed in theanalysis. For the polyethylene case, recycle velocities of 35 and 55ft/sec were considered, with a liquid condensation level of 7.65 wt %.The polypropylene cases were at 15.6 wt % and 24.9% condensing, eachcomparing velocities of 25 and 35 ft/sec.

In this analysis the vapor recovery through the collection device wasmaintained below 21% of the incoming flow rate. This amount of bypassaround the Reactor distributor plate may be tolerated without disruptingthe fluidization of the polymer bed.

When comparing the PE results at increasing recycle line velocities,about the same liquid droplet collection efficiency was observed. Forthe polyethylene system, as the mean droplet size was increased from 104to 312 microns, the liquid droplet recovery increased by about 4%. Thepolypropylene system has a smaller difference in density between thevapor and liquid phases. Accordingly the momentum difference is smallerbetween the phases. The improvement in liquid collection efficiency withincreasing liquid droplet size is smaller.

The analysis reveals that it is feasible to collect about 20% to about29% of the liquid into the smaller split stream with a differentialpressure—that is, the difference in pressure between the points ofentrance into the reactor of the smaller split stream and the largerstream of line 3—of less than 3 psi. Liquid collection efficiency isstrongly correlated with droplet size and momentum difference betweenthe vapor and liquid phases.

Splitter Design

As indicated above, the splitter is preferably designed more or less asillustrated in FIGS. 2, 3, 9. 10, or 11 using an elbow having a smallline 19 on its large radius side and most preferably a settling chamber25 defined by a take-off of larger diameter than the small line 19. Theconstriction to line 19 can be perpendicular to flow as wall 29 in FIG.3 or somewhat tapered or gradual as in FIG. 2, and line 19 may berectangular in cross section as seen in FIG. 11 a. As seen in thevelocity and droplet distribution data above, use of a settling chamberwill result in an accumulation of liquid available to be discharged withthe slip stream, giving it a higher ratio of liquid to gas than theincoming recycle stream or the primary recycle stream. The designillustrated in FIG. 3, wherein the settling chamber has a verticaldead-end wall 29, generates a slip stream in line 19 relatively highlyenriched in liquid but relatively low in volume; tapering wall 29 as inFIG. 2 will result in a higher volume of fluid having a lower liquidpercentage.

Referring to FIG. 12. parallel discharge tanks, which include conduitsand valves, may be intermittently operated in an alternate sequentialmode as disclosed by Aronson in U.S. Pat. No. 4,621,952. Theinterconnecting conduits, isolation valves and vents to the upper regionof the fluidized bed are operated to minimize the loss of valuable gaswith the product solid.

During a discharge process, the polymer product and raw material fluidsexit through either lines 8 or 8′ into either vessel 10 or 10′. Thisproduct removal obtained by opening either valves 9 or 9′ when thedownstream vessel is initially at a lower pressure than the reactorvessel 1. The choice for product withdraw vessel as 10 or 10′ isalternated, while the parallel vessel serves as a pressure equalizationvessel. Thus the discharge cycle vessel 10 receives the solid and fluidfrom the reactor 1. Vessel 10′ contains substantially no solid at thispoint. It is available to receive the subsequent pressure equalizationvent from vessel 10.

For one discharge cycle, the polymer product and fluids from thereaction vessel 1 enter the active discharge vessel 10. The vent valve65 in conduit 62 interconnecting vessel 10 with the upper portion of thefluidized bed 2 is open, thus allowing fluid to return to the reactor 1.Valves 9 and 65 are then closed to isolate vessel 10 from the reactionvessel 1 at pressure approximately equal to that in the reaction vessel.The polymer product settles into vessel 10. The valve 66 in the conduit61 connecting with the parallel vessel 10′ is opened and the pressuresallowed to equalize between the vessels 10 and 10′. Valve 61interconnecting vessels 10 and 10′ is then closed. Fluid is thustransferred from vessel 10 to vessel 10′ during the pressureequalization. Upon a subsequent alternate discharge cycle into vessel10′ a portion of that equalization fluid is vented back into thereaction vessel 1. This venting and pressure equalization provides anattractive mechanism for the recovery of valuable unreacted monomers andinert materials from the semi-batch discharge system without the use ofmechanical equipment such as compressors and pumps.

The polymer product is then transferred from vessel 10 by gravity andpressure equalization into vessel 12. Valve 11 is then closed to isolatevessel 12 from vessel 10. After the polymer product settles into vessel12, the valve 63 in the conduit 60 connecting with the parallel vessel12′ is opened and the pressures allowed to equalize between the vessels12 and 12′. Fluid was thus transferred from vessel 12 to vessel 12′during the pressure equalization. Upon a subsequent discharge cycle intovessel 10′ and polymer product transfer into vessel 12′ a portion ofthat equalization fluid is vented back into the discharge vessel 10′. Onthe next discharge event into vessel 10′ a portion of the fluid invessel 10′ vents back to the reactor vessel 1 through the conduit 64′and valve 65′.

After additional equalization venting and polymer product transfer stepsthrough conduit 14, the polymer product and remaining fluids areprocessed by downstream equipment. Since the fluids contain valuable rawmaterials, it is economically desirable to minimize their net removalfrom the reactor vessel 1. The product removal capacity of theproduction facility is thus determined by the time necessary toeconomically complete a discharge and pressure equalization cycle, andthe overall production capacity of the reactor system may be limited bythe efficiency of product removal. This is especially so as higher andhigher condensing ratios are utilized to improve the rate of removal ofthe process heat of reaction. The raw material efficiency of the processis affected by the minimum pressure which is obtainable by the steps ofvessel pressure equalization. Achieving a lower equalization pressureaffords a reduction in the raw material usage of the facility.

Operation of the reaction system with a partially condensed recyclethrough conduit 3 can result in liquid being present in the lowerregions of the fluidized bed 2. When substantial amounts of a liquidphase are also intermittently removed with the solid and gas from thefluidized bed 2 to the discharge vessels 10 and 10′, the time durationof the discharge process steps can increase. This is because as thepressure is reduced during the venting and pressure equalization stagesof the discharge process, a portion of the liquid phase will vaporize.This liquid vaporization increases the final pressure and the timenecessary to complete all the venting and pressure equalization stepsbetween the discharge vessels and downstream processing equipment. Theincrease in time to complete the necessary steps of the semi-batchdischarge process, reduces the product removal capacity of the facility.

The cycle time of the semi-batch discharge process can be reduced byinhibiting the discharge vessel venting steps from reaching pressureequalization. However, if the pressure is not equalized, then a largerportion of the discharged fluids will leave the reaction system. Thus alarger quantity of fluid materials is lost or must be processed bydownstream operations for recovery. The raw material efficiency of theprocess is effected by the minimum pressure which is obtainable by thesteps of vessel venting and pressure equalization. Achieving a lowerequalization pressure affords a reduction in the raw material usage ofthe production facility. The optimal product removal capacity of thefacility is thus determined by the time necessary to economicallycomplete a discharge and pressure equalization cycle.

In the practice of this invention conventional pressure measurement andtransmitter devices are installed on the discharge vessels 10, 10′, 12and 12′. An increase in equalization pressure of the discharge vesselsis observed as the liquid content in streams 8 and 8′ is increased. Anyvapor liquid separating equipment can be used to reduce the liquidcontent of the recycle stream 3. It is well known by those skilled inthe art of vapor liquid separation that mechanical devices such asimpingement demisters and hydrocyclones can be used to accomplish vaporliquid separation.

In the preferred embodiment of this invention the vapor liquidseparation of the recycle stream exiting the cycle gas cooler 17 isachieved using the inertial bypass stream 19. During “condensed mode”operation, when the equalization pressure in the discharge vessels isobserved to increase, the bypass valve 21 is opened and the flowrate ofenriched liquid bypass stream 19 is increased while the liquid contentof the recycle stream 3 is reduced. This serves to reduce the liquidcontent in the polymer product discharge streams 8 and 8′. The magnitudeof the pressure being measured in the discharge vessels 10, 10′, 12, 12′is thereby reduced and the rate of product transfer is thereby improved.

This invention reduces the quantity of liquid which can exit thefluidized bed 2 with the polymer product into vessels 10 and 10′ whenoperating in the condensed mode. The bypass streams can be returned tothe reactor fluidized bed 2, at an elevation above that of the polymerproduct discharge lines 8 and 8′. The separation of the recycle streamis conducted such that the liquid content of the bypass streams areenriched and the liquid content of the primary recycle stream 3 isreduced. By opening valve 21 and bypassing a liquid enriched portion ofthe recycle flow into stream 19, a lesser quantity of liquid is returnedto the bottom of the reactor through stream 3. Reducing the liquidcontent of the recycle stream 3 to the reactor inlet also reduces theliquid content in the fluid mixture exiting from the reactor with thesolid polymer product in the discharge streams 8 and 8′.

By practicing this invention with “condensed mode” of operation both theeconomic penalties of longer duration semi-batch discharge cycles and ofincreased raw material usage are reduced. Returning the enriched liquidcontent bypass streams 19 and 20 to the fluidized bed 2, at a locationabove the product removal streams 8 and 8′ retains the heat removalproduction advantages of “condensed mode” operation.

Compared in FIG. 13 are dry and wet discharge events for identicaldischarge vessels which are operated at equivalent source vessel andsink vessel initial pressure. The wet discharge includes with the solidpolymer product a vapor and liquid fluid mixture which comprises 24.66weight percent liquid. The final equalization pressure is higher for thewet discharge event as compared with the dry discharge. The timeduration to equalize pressure for the wet discharge event is longer thanfor the dry discharge event. The product removal capacity is therebylower and the raw material loss higher when liquid is discharged alongwith the vapor and solid flow.

EXAMPLES

In order to provide a better understanding of the present invention thefollowing examples are provided. They represent computer simulations ofcommercial scale operating facilities for the gas phase fluidized bedexothermic production of polyethylene and polypropylene.

Example 1

A fluidized bed polymerization reactor system similar to that of FIG. 1is producing linear low density polyethylene which contains about 10weight percent copolymerized hexene. Referring to FIG. 1, the recyclestream in line 4 contains a mixture of hydrogen, nitrogen, methane,ethane, ethylene, hexene and hexane. The pressure 374.4 psia andtemperature 53.0° C. at the exit of the external cooler 17 yields a bulkrecycle stream containing 7.65 weight percent liquid. The entraineddroplet size of the liquid is in the range of 104 to 312 microns or0.0041 to 0.0123 inches. The liquid phase is rich in hexene and hexaneand has a density of 38.6 lb/ft³. The vapor phase is rich in the morevolatile components of the mixture and has a density of 1.80 lb/ft³. Asillustrated in FIG. 1 the small bypass or slip stream in line 19 isdesigned to contain 19.59 percent of the inlet vapor (the gas phase ofthe inlet fluid) with a 0.9 psi differential pressure. The velocity ofthe fluid is 35 feet per second at the exit of the cooler 17 andupstream of the separation elbow. The small stream 19 contains anaverage of 25.95 percent of the liquid entering the inlet. The averageenrichment is thus 1.29 times the liquid content of the bulk recyclestream exiting the cooler 17.

Example 2

A fluidized bed polymerization reactor is producing linear low densitypolyethylene which contains about 10 weight percent polymerized hexene.Referring to FIG. 1, the recycle stream 4 contains a mixture ofhydrogen, nitrogen, methane, ethane, ethylene, hexene and hexane. Thepressure 374.4 psia and temperature 53.0° C. at the exit of the externalcooler 17 yields a recycle stream containing 7.65 weight percent liquid.The liquid phase, rich in hexene and hexane, has a density of 38.6lb/ft³. The vapor phase is rich in the more volatile components of themixture and has a density of 1.80 lb/ft³. As illustrated in FIG. 1, thesmall stream 19 is designed to contain 19.61 percent of the inlet vaporwith a 2.3 psi differential pressure. The velocity of the recycle streamis 55 feet per second at the exit of the cooler 17 and upstream of theseparation elbow. The small stream 19 contains an average of 26.61percent of the inlet liquid. The average weight ratio enrichment of thesmall stream 19 is thus 1.33 times the liquid content of the bulkrecycle stream exiting the cooler 17.

Example 3

A fluidized bed polymerization reactor is producing propylenehomopolymer. Referring to FIG. 1, the recycle stream 4 contains amixture of hydrogen, nitrogen, propylene and propane. The pressure is556.2 psia and temperature of 63.1° C. at the exit of the externalcooler 17 yield a recycle stream containing 15.60 weight percent liquid.The liquid phase, rich in propylene and propane, has a density of 26.4lb/ft³. The vapor phase is rich in the more volatile components of themixture and has a density of 4.80 lb/ft³. As illustrated in FIG. 1 thesmall stream 19 is designed to contain 17.38 percent of the inlet vaporwith a 0.9 psi differential pressure. The fluid velocity is 25 feet persecond at the exit of the cooler 17 and upstream of the separationelbow. The small stream 19 contains an average of 21.3 percent of theinlet liquid. The average weight ratio enrichment is thus 1.18 times theliquid content of the bulk recycle stream exiting the cooler 17.

Example 4

A fluidized bed polymerization reactor is producing propylenehomopolymer. Referring to FIG. 1, the recycle stream 4 contains amixture of hydrogen, nitrogen, propylene and propane. The pressure(556.2 psia) and temperature (63.1° C.) at the exit of the externalcooler 17 yield a recycle stream containing 15.60 weight percent liquid.The liquid phase, being rich in propylene and propane, has a density of26.4 lb/ft³. The vapor phase is rich in the more volatile components ofthe mixture and has a density of 4.80 lb/ft³. As illustrated in FIG. 1the small stream 19 is designed to contain 17.28 percent of the inletvapor with a 1.7 psi differential pressure. The fluid velocity is 35feet per second at the exit of the cooler 17 and upstream of theseparation elbow. The small stream 19 contains an average of 21.4percent of the inlet liquid. The average weight ratio enrichment is thus1.19 times the liquid content of the bulk recycle stream exiting thecooler 17.

Example 5

A fluidized bed polymerization reactor is producing propylenehomopolymer. Referring to FIG. 1, the recycle stream 4 contains amixture of hydrogen, nitrogen, propylene and propane. The pressure 501.2psia and temperature 57.2° C. at the exit of the external cooler 17yields a recycle stream containing 24.96 weight percent liquid. Theliquid phase being rich in propylene and propane has a density of 27.3lb/ft. The vapor phase, being rich in the more volatile components ofthe mixture, has a density of 4.14 lb/ft³. As illustrated in FIG. 1 thesmall branch stream 19 is designed to contain 18.45 percent of the inletvapor with a 0.9 psi differential pressure. The fluid velocity is 25feet per second at the exit of the cooler 17 and upstream of theseparation elbow. The small stream 19 contains an average of 22.60% ofthe inlet liquid. The average weight ratio enrichment is thus 1.16 timesthe liquid content of the bulk recycle stream exiting the cooler 17.

Example 6

A fluidized bed polymerization reactor is producing propylenehomopolymer. Referring to FIG. 1, the recycle stream 4 contains amixture of hydrogen, nitrogen, propylene and propane. The pressure of501.2 psia and temperature 57.2° C. at the exit of the external cooler17 yields a recycle stream containing 24.96 weight percent liquid. Theliquid phase, being rich in propylene and propane, has a density of 27.3lb/ft³. The vapor phase being rich in the more volatile components ofthe mixture, has a density of 4.14 lb/ft³. As illustrated in FIG. 1 thesmall branch stream 19 is designed to contain 18.44 percent of the inletvapor with a 1.7 psi differential pressure. The fluid velocity is 35feet per second at the exit of the cooler 17 and upstream of theseparation elbow. The small stream 19 contains an average of 22.95percent of the inlet liquid. The enrichment is thus 1.17 times theliquid content of the bulk recycle stream exiting the cooler 17.

Example 7

A 3 dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an abrupt 90 degree elbow. A 12 inch internaldiameter slip stream cylindrical pipe was located across from the bottomof the inlet pipe as referred in FIG. 9. At the inlet to the domain1,000 liquid droplets were injected throughout the cross section. Thefraction of droplets exiting through the secondary bypass slip streamwas compared with the fraction exiting the primary outlet. The fractionof vapor exiting the bypass and primary outlet were also compared. Theweight fraction of liquid exiting the bypass was computed by materialbalance and compared with the weight fraction of liquid at the inlet.Operating conditions were identical to those shown in Example 1. Thesecondary bypass slip stream was designed to contain 16.17 percent ofthe inlet vapor. This design required a 1.0 psi. pressure differencebetween the primary outlet and the secondary bypass slip stream. Thisoutlet differential pressure was at an inlet fluid velocity of 35 feetper second. The secondary bypass slip stream was computed to contain anaverage of 25.81 percent of the total inlet liquid. The average weightratio enrichment was 1.51 times the weight fraction of liquid at theinlet.

Example 8

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an abrupt 90 degree elbow. A 12 inch internaldiameter slip stream cylindrical pipe was located across from the top ofthe inlet pipe as referred in FIG. 10. At the inlet to the domain 1,000liquid droplets were injected throughout the cross section. The fractionof droplets exiting through the secondary bypass slip stream wascompared with the fraction exiting the primary outlet. The fraction ofvapor exiting the bypass and primary outlet were also compared. Theweight fraction of liquid exiting the bypass was computed by materialbalance and compared with the weight fraction of liquid at the inlet.Operating conditions were identical to those shown in Example 1. Thesecondary bypass slip stream was designed to contain 15.25 percent ofthe inlet vapor. This design required a 1.0 psi pressure differencebetween the primary outlet and the secondary bypass slip stream. Thisoutlet differential pressure was at an inlet fluid velocity of 35 feetper second. The secondary bypass slip stream was computed to contain anaverage of 29.07 percent of the total inlet liquid. The average weightratio enrichment was 1.77 times the weight fraction of liquid at theinlet.

Example 9

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 1. The secondary bypass slip stream was designed to contain20.70 percent of the inlet vapor. This design required a 0.7 psipressure difference between the primary outlet and the secondary bypassslip stream. This outlet differential pressure was at an inlet fluidvelocity of 35 feet per second. The secondary bypass slip stream wascomputed to contain an average of 36.88 percent of the total inletliquid. The average weight ratio enrichment was 1.69 times the weightfraction of liquid at the inlet.

Example 10

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B116.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 2. The secondary bypass slip stream was designed to contain20.79 percent of the inlet vapor. This design required a 1.8 psipressure difference between the primary outlet and the secondary bypassslip stream. This outlet differential pressure was at an inlet fluidvelocity of 55 feet per second. The secondary bypass slip stream wascomputed to contain an average of 38.59 percent of the total inletliquid. The average weight ratio enrichment was 1.73 times the weightfraction of liquid at the inlet.

Example 11

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. A 18 inch wideby 9 inch high rectangular slip stream duct was located across from thecenter streamline of the inlet pipe as referred in FIG. 11. At the inletto the domain 1,000 liquid droplets were injected throughout the crosssection. The fraction of droplets exiting through the secondary bypassslip stream was compared with the fraction exiting the primary outlet.The fraction of vapor exiting the bypass and primary outlet were alsocompared. The weight fraction of liquid exiting the bypass was computedby material balance and compared with the weight fraction of liquid atthe inlet. Operating conditions were identical to those shown in Example3. The secondary bypass slip stream was designed to contain 18.12percent of the inlet vapor. This design required a 0.6 psi pressuredifference between the primary outlet and the secondary bypass slipstream. This outlet differential pressure was at an inlet fluid velocityof 25 feet per second. The secondary bypass slip stream was computed tocontain an average of 35.38 percent of the total inlet liquid. Theaverage weight ratio enrichment was 1.67 times the weight fraction ofliquid at the inlet.

Example 12

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 4. The secondary bypass slip stream was designed to contain18.15 percent of the inlet vapor. This design required a 1.3 psipressure difference between the primary outlet and the secondary bypassslip stream. This outlet differential pressure was at an inlet fluidvelocity of 35 feet per second. The secondary bypass slip stream wascomputed to contain an average of 30.10 percent of the total inletliquid. The average weight ratio enrichment was 1.49 times the weightfraction of liquid at the inlet.

Example 13

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 5. The secondary bypass slip stream was designed to contain19.71 percent of the inlet vapor. This design required a 0.7 psipressure difference between the primary outlet and the secondary bypassslip stream. This outlet differential pressure was at an inlet fluidvelocity of 25 feet per second. The secondary bypass slip stream wascomputed to contain an average of 40.09 percent of the total inletliquid. The average weight ratio enrichment was 1.59 times the weightfraction of liquid at the inlet.

Example 14

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 1. Variations in droplet size were considered to discern thelimiting droplet size, below which there was no enrichment of liquidinto the secondary bypass slip stream. The secondary bypass slip streamwas designed to contain 20.70 percent of the inlet vapor. This designrequired a 0.7 psi pressure difference between the primary outlet andthe secondary bypass slip stream. This outlet differential pressure wasat an inlet fluid velocity of 35 feet per second. For a liquid dropletsize larger than 20 microns, the secondary bypass slip stream wascomputed to contain more liquid than the inlet recycle pipe. Thus liquidenrichment into the secondary bypass stream was obtained when the inletdroplet size exceeded 20 microns.

Example 15

A 3-dimensional cylindrical recycle pipe with a 40.25 inch internaldiameter was analyzed with an ASME B16.9 specified elbow. An 18 inchwide by 9 inch high rectangular slip stream duct was located across fromthe center streamline of the inlet pipe as referred in FIG. 11. At theinlet to the domain 1,000 liquid droplets were injected throughout thecross section. The fraction of droplets exiting through the secondarybypass slip stream was compared with the fraction exiting the primaryoutlet. The fraction of vapor exiting the bypass and primary outlet werealso compared. The weight fraction of liquid exiting the bypass wascomputed by material balance and compared with the weight fraction ofliquid at the inlet. Operating conditions were identical to those shownin Example 3. Variations in droplet size were considered to discern thelimiting droplet size, below which there was no enrichment of liquidinto the secondary bypass slip stream. The secondary bypass slip streamwas designed to contain 18.12 percent of the inlet vapor. This designrequired a 0.6 psi pressure difference between the primary outlet andthe secondary bypass slip stream. This outlet differential pressure wasat an inlet fluid velocity of 25 feet per second. For a liquid dropletsize larger than 47 microns, the secondary bypass slip stream wascomputed to contain more liquid than the inlet recycle pipe. Thus liquidenrichment into the secondary bypass stream was obtained when the inletdroplet size exceeded 47 microns.

Example 16

A fluidized bed polymerization reactor similar to that of FIG. 1 isproducing propylene homopolymer. Referring to FIG. 1, the recycle stream4 contains a mixture of hydrogen, nitrogen, propylene and propane. Thepressure is 501.2 psia and the temperature is 57.2° C. at the exit ofcooler 17, yielding a recycle stream containing 24.96 weight percentliquid at the cooler exit. The liquid phase is rich in propylene andpropane and has a density of 27.3 lb/ft{circumflex over ( )}3. The vaporphase is rich in the more volatile species and has a density of 4.14lb/ft{circumflex over ( )}3. As illustrated in FIG. 1, the small line 19is designed to contain 19.71 percent of the inlet vapor with a 0.7 psidifferential pressure between the primary outlet and the secondarybypass. The recycle stream velocity is 25 feet per second at the exit ofthe cooler 17 and upstream of the splitter 22. When the valve 21 isclosed in bypass stream 19, some amount of liquid is removed fromfluidized bed 2 as the polymer product is withdrawn through line 8 intovessel 10. The liquid vaporizes in vessel 10, which increases thepressure in vessel 10. This back pressure inhibits the flow of polymerproduct through line 8. The polymer production capacity of the facilityis thus limited by the ability to remove polymer product through line 8.Opening the valve 21 in the bypass line 19 causes the amount of liquidentering the fluidized bed 2 through line 3 to be reduced. The secondarybypass slip stream (line 19) was computed to contain an average of 40.09percent of the total inlet liquid. Bypass line 19 achieves an enrichmentof liquid content of 1.59 times the liquid content of the stream exitingthe cooler 17, and the production and removal of polymer product isincreased by 18%.

Example 17

A fluidized bed polymerization reactor with parallel and interconnecteddischarge tanks similar to that of FIG. 12 was producing propylenehomopolymer. The recycle stream 4 at a pressure of 490 psia, contained amixture of hydrogen, nitrogen, propylene and propane. The pressure atthe discharge lines 8 and 8′ was 492.6 psia. The pressure was 501.2 psiaat the exit of the external cooler 17. At a temperature below the dewpoint of 61° C. there was liquid present in stream 67, at the exit ofthe cooler 17.

When operating in “condensed mode”, with the recycle stream 3 containingsome liquid, then a portion of the liquid was carried from fluidized bed2 as the polymer product was intermittently withdrawn through streams 8and 8′. For this example, streams 8 and 8′ were located less than 2 feetabove the top elevation of the distributor plate 7. We have found thatthe relative concentration of liquid to vapor in the discharge streams 8and 8′ exceeds that in stream 3. Comparisons are made in Table 6 withincreasing levels of liquid in stream 8 or 8′, which enters thedischarge vessels 10 or 10′. Discharge conditions and the resulting timeduration of the discharge process are displayed in Table 6.

For the test cases of this Example 17, the two phase fluid recyclestream exiting the cooler 17 was separated into a primary recycle stream3 and a bypass stream 19. The bypass stream was returned to the reactorfluidized bed 2, at an elevation above that of the polymer productdischarge streams 8 and 8′. The separation was conducted such that theliquid content of the bypass stream was enriched and the liquid contentof the primary recycle stream was reduced. The test cases with reducedliquid at the reactor inlet, resulted in reduced liquid in the fluidmixture exiting with the solid polymer product in the discharge streams8 and 8′.

During a discharge process, the polymer product and raw material fluidsexit through either line 8 or 8′ into either vessel 10 or 10′. Thisproduct removal obtained by opening either valve 9 or 9′ when thedownstream vessel 10 or 10′ is initially at a lower pressure than thereactor vessel 1. During the solid and fluid transfer into the vessel, avent valve 65 or 65′ is opened, allowing a portion of the fluid in thevessel to be returned to the reactor vessel 1.

The choice for product withdraw vessel as 10 or 10′ is alternated. Theparallel vessel, which did not receive the polymer product, wouldsubsequently serve as a pressure equalization vessel with the vent fromthe discharge vessel. The time duration to equalize and final pressurewas determined and recorded in Table 6.

A small amount of absorbed gas is also evolved from the solid polymerproduct during the discharge process. The quantity of gas dissolved inthe solid product and evolved during the discharge process is dependentupon the crystallinity, particle size, molecular weight and Theologicalproperties of the polymer product, the physical properties of theabsorbate, and the composition, temperature and pressure of thesurrounding fluid.

For the four test cases in this Example 17 the polymer product had asolid phase density of 56.4 lb/ft{circumflex over ( )}3. For a givensemi-batch discharge cycle, the solid polymer product occupied a volumeof 17.17 ft{circumflex over ( )}3 in the vessels. Each discharge eventremoved 968 lb of solid polymer product from the reactor fluidized bed2. The balance of volume was occupied by raw material fluids. Thesefluids were in the vapor and optionally liquid phase. They comprisedunreacted monomers and inert species. The total volume in theinterconnecting system for vessels 10 and 10′ was 94% of the availablevolume for vessels 12 and 12′.

The total time duration of the discharge process is exemplified in Table6. Cases with multiple venting and equalization steps are included. Timeduration for valves to open and close are included in each event. Thetotal time duration includes several sequential batch steps. The firststep is the polymer discharge and venting into vessel 10. The pressureequalization with vessel 10′ was the second step. The third step was thetransfer of polymer and fluids into vessel 12. This step includes theventing and pressure equalization between vessels 10 and 12. The ventequalization between vessel 12 and 12′ was the fourth step in thedischarge process.

The venting and transfer though lines 14 or 14′ to downstream processingequipment was the final step in the discharge process. In all test casesof this Example a vent through lines 14 or 14′ was included to lower thepressure in vessel 12 or 12′ to 140 psia, prior to the transfer of thesolid polymer product. The equipment into which the venting was directedand transfer was completed had an available volume of 27 times theavailable volume in vessel 10.

The product removal capacity of the production facility is limited bythe time necessary to economically complete a discharge and pressureequalization cycle. When operating in “condensed mode”, with thedischarge of fluid containing some liquid, the time duration necessaryto complete the intermittent discharge process was shown to increase asthe quantity of liquid in the discharge streams 8 and 8′ was increased.

In this Example 17 the product removal capacity of the discharge systemdecreased from 17,744 to 16,933 lb polymer per hour as the liquidconcentration in streams 8 or 8′ increased from 15.59 to 29.61 weightpercent.

Also included in Table 6 is a summary of the quantity of raw materialmass transferred during the pressure equalization steps. Raw materialmass, which was not transferred to a parallel vessel by equalization,leaves the discharge system with the solid polymer product. Foreconomical operation a portion of these fluids must be processed forrecovery to the reaction system. The raw material efficiency waseffected by the minimum pressure obtained by the steps of vesselpressure equalization. At increasing final pressure of the fourth step,a larger quantity of the valuable raw materials exits with the solidpolymer product in streams 14 or 14′. These fluid materials are lost ormust be processed by downstream operations for recovery.

The “removal ratio” was defined at the mass of raw material removedrelative to the mass of solid polymer product. The mass of raw materialincludes the initial vent through lines 14 or 14′ along with fluidtransported with the solid polymer product through lines 14 or 14′. This“removal ratio” was shown to increase with liquid quantity in thedischarge. The cost of recovering or losing valuable raw materialincreases with liquid quantity in the discharge. In this Example 17 the“removal ratio” increased from 0.1182 to 0.1481 lb fluid per lb ofpolymer as the liquid concentration in streams 8 or 8′ increased from15.59 to 29.61 weight percent. A high liquid content in streams 8 and 8′is clearly undesirable. TABLE 6 PROPYLENE HOMPOLYMER DISCHARGE CYCLECOMPARISON OF LIQUID CONTENT IN DISCHARGE STREAMS 8 AND 8′ Case 1 2 3 4Stream 67 Cooler Outlet Liquid 24.96 24.96 24.96 24.96 (weight %) Stream8 Discharge Fluid Excluding Solids Vapor Density 3.99 4.05 4.10 4.14(lb/ft{circumflex over ( )}3) Liquid Density 27.58 27.46 27.34 27.24(lb/ft{circumflex over ( )}3) Liquid 29.61 24.66 19.90 15.59 (weight %)Profile Vessel Pressure (psia) 10 After 490.0 490.0 490.0 490.0Discharge 10 Equalization 397.7 388.6 386.8 371.4 with 10′ 12, 12′Before 20.0 20.0 20.0 20.0 Transfer 10 After 299.9 287.9 285.4 265.1Transfer to 12 12 Equalization 206.9 197.3 195.6 178.2 with 12′ 12 AfterVent, 20.0 20.0 20.0 20.0 Transfer Pressure Equalization Recovered Mass(lb) 10 Equalize 77.8 81.6 82.5 88.8 with 10′ 10 Transfer 79.2 78.3 78.778.3 to 12 12 Equalize 77.7 73.7 73.0 65.7 with 12′ Stream 14 Vent andTransfer Solid with Fluid to Downstream Processes Vessel 12 53.4 45.043.9 28.0 Vent (lb) Solid Product 968. 968. 968. 968. (lb) Fluid with90.0 94.2 89.3 86.4 Solid (lb) Stream 14 Removal Ratio (Vent + Fluid)/0.1481 0.1438 0.1376 0.1182 Solid Profile Time Duration (seconds) 10Discharge 38.2 38.1 38.0 38.0 and Vent 10 Equalize 34.6 35.2 35.3 36.2with 10′ 10 Transfer 43.8 43.8 43.7 43.7 to 12 12 Equalize 30.2 29.829.6 28.5 with 12′ 12 Vent and 59.0 56.5 56.0 50.0 Transfer Total Cycle205.8 203.4 202.6 196.4 Duration Product 16,933. 17,133. 17,200. 17,744.Discharge (lb/hr)

Example 18

A fluidized bed polymerization reactor with parallel and interconnecteddischarge tanks similar to that of FIG. 12 was producing linear lowdensity polyethylene which contained about 10 weight percent polymerizedcopolymer hexene. The 50 recycle stream 4 at a pressure of 364.7 psia,contained a mixture of hydrogen, nitrogen, methane, ethane, ethylene,hexene and hexane. The pressure at the discharge line 8 and 8′ was 369.0psia. The pressure was 374.4 psia at the exit of the external cooler 17.At a temperature below the dew point of 68.7° C. there was liquidpresent in stream 67, at the exit of the cooler 17.

When operating in “condensed mode”, with the recycle stream 3 containingsome liquid, then a portion of the liquid was carried from fluidized bed2 as the polymer product was intermittently withdrawn through streams 8and 8′. For this example, streams 8 and 8′ were located less than 2 feetabove the top elevation of the distributor plate 7. We have found thatthe relative concentration of liquid to vapor in the discharge streams 8and 8′ exceeded that in stream 3. Comparisons are made in Table 7 withincreasing levels of liquid in streams 8 or 8′, which enters thedischarge vessels 10 or 10′. Discharge conditions and the resulting timeduration of the discharge process are displayed in Table 7.

For the test cases of this Example 18, the two phase fluid recyclestream exiting the cooler 17 was separated into a primary recycle stream3 and a bypass stream 19. The bypass stream was returned to the reactorfluidized bed 2, at an elevation above that of the polymer productdischarge streams 8 and 8′. The separation was conducted such that theliquid content of the bypass stream was enriched and the liquid contentof the primary recycle stream was reduced. The test cases with reducedliquid at the reactor inlet, resulted in reduced liquid in the fluidmixture exiting with the solid polymer product in the discharge streams8 and 8′.

Example 17 includes a more detailed discussion of the discharge processsteps, the results of which are summarized in Table 7 for this Example18.

For the four test cases in this Example 18 the polymer product had asolid phase density of 57.3 lb/ft{circumflex over ( )}3. For a givensemi-batch discharge cycle, the solid polymer product occupied a volumeof 20.02 ft{circumflex over ( )}3 in the vessels. Each discharge eventremoved 1,147 lb of solid polymer product from the reactor fluidized bed2. The balance of volume was occupied by raw material fluids. Thesefluids were in the vapor and optionally liquid phase. They comprisedunreacted monomers, comonomers and inert species. The total volume inthe interconnecting system for vessels 10 and 10′ was 102% of theavailable volume for vessels 12 and 12′.

The total time duration of the discharge process is illustrated in Table7. Cases with multiple venting and equalization steps are included. Timeduration for valves to open and close are included in each event. Thetotal time duration includes several sequential batch steps. These wereidentified in Example 17.

To complete the discharge process, the solid polymer product and someremaining raw material fluids were transferred through either lines 14or 14′, to downstream processing equipment. The first vessel equipmentinto which vent was taken and transfer was completed had an availablevolume of 6 times the available volume in vessel 10.

The product removal capacity of the production facility was limited bythe time necessary to economically complete a discharge and pressureequalization cycle. When operating in “condensed mode”, with thedischarge of fluid containing some liquid, the time duration necessaryto complete the intermittent discharge process was shown to increase asthe quantity of liquid in the discharge streams 8 and 8′ was increased.

In this Example 18 the product removal capacity of the discharge systemdecreased from 26,118 to 25,954 lb polymer per hour as the liquidconcentration in streams 8 or 8′ increased from 4.29 to 16.77 weightpercent.

Also included in Table 7 is a summary of the quantity of raw materialmass transferred during the pressure equalization steps. Raw materialmass, which was not transferred to a parallel vessel by equalization,leaves the discharge process with the solid polymer product. Foreconomical operation a portion of these fluids must be processed forrecovery to the reaction system. The raw material efficiency wasaffected by the minimum pressure obtained by the steps of vesselpressure equalization. At increasing final pressure, a larger quantityof the valuable raw materials exits with the solid polymer product instreams 14 or 14′. These fluid materials are lost or must be processedby downstream operations for recovery.

The “removal ratio” was shown to increase with liquid quantity in thedischarge. In this Example 18 the “removal ratio” increased from 0.0138to 0.0161 lb fluid per lb of polymer as the liquid concentration instreams 8 or 8′ increased from 4.29 to 16.77 weight percent. High liquidconcentrations in streams 8 and 8′ are not desirable. TABLE 7POLYETHYLENE HEXENE COPOLYMER DISCHARGE CYCLE COMPARISON OF LIQUIDCONTENT IN DISCHARGE STREAMS 8 AND 8′ Case 1 2 3 4 Stream 67 CoolerOutlet Liquid 15.25 15.25 15.25 15.25 (weight %) Stream 8 DischargeFluid Excluding Solids Vapor Density 1.93 1.86 1.76 1.76(lb/ft{circumflex over ( )}3) Liquid Density 40.78 40.26 38.66 38.12(lb/ft{circumflex over ( )}3) Liquid 16.77 15.07 7.56 4.29 (weight %)Profile Vessel Pressure (psia) 10 After 364.7 364.7 364.7 364.7Discharge 10 Equalization 249.2 245.6 246.9 245.7 with 10′ 12, 12′Before 20.0 20.0 20.0 20.0 Transfer 10 After 159.2 154.3 155.3 154.1Transfer to 12 12 Equalization 89.2 85.4 84.4 83.8 with 12′ 12 AfterFinal 20.0 20.0 20.0 20.0 Transfer Pressure Equalization Recovered Mass(lb) 10 Equalize 36.1 35.0 33.8 33.6 with 10′ 10 Transfer 27.7 26.5 25.725.6 to 12 12 Equalize 20.5 19.4 19.1 18.9 with 12′ Stream 14 TransferSolid with Fluid to Downstream Processes Solid Product 1,147. 1,147.1,147. 1,147. (lb) Fluid with 18.5 17.0 16.0 15.8 Solid (lb) Stream 14Removal Ratio Fluid/Solid 0.0161 0.0148 0.0139 0.0138 Profile TimeDuration (seconds) 10 Discharge 22.1 22.1 22.1 22.1 and Vent 10 Equalize25.3 24.9 24.8 24.6 with 10′ 10 Transfer 36.1 36.1 36.0 36.0 to 12 12Equalize 14.7 14.6 14.8 14.7 with 12′ 12 Final 60.9 60.8 60.8 60.7Transfer Total Cycle 159.1 158.5 158.5 158.1 Duration Product 25,954.26,051. 26,051. 26,118. Discharge (lb/hr)

Example 19

A fluidized bed polymerization reactor with parallel and interconnecteddischarge tanks similar to that of FIG. 12 was producing linear lowdensity polyethylene which contained about 9 weight percent polymerizedcopolymer butene. The recycle stream 4 at a pressure of 364.7 psia,contained a mixture of hydrogen, nitrogen, methane, ethane, ethylene,butene, butane and isopentane. The isopentane being added to promotepartial condensation of the recycle stream. The pressure at thedischarge line 8 and 8′ was 369.0 psia. The pressure was 375.2 psia atthe exit of the external cooler 17. At a temperature below the dew pointof 65.4° C. there was liquid present in stream 67, at the exit of thecooler 17.

When operating in “condensed mode”, with the recycle stream 3 containingsome liquid, then a portion of the liquid was removed from fluidized bed2 as the polymer product was intermittently withdrawn through streams 8and 8′. For this example, streams 8 and 8′ were located less than 2 feetabove the top elevation of the distributor plate 7. We have found thatthe relative concentration of liquid to vapor in the discharge streams 8and 8′ exceeds that in stream 3. Comparisons are made in Table 8 withincreasing levels of liquid in streams 8 or 8′, which enters thedischarge vessels 10 or 10′. Discharge conditions and the resulting timeduration of the discharge process are displayed in Table 8.

For the test cases of this Example 18, the two phase fluid recyclestream exiting the cooler 17 was separated into a primary recycle stream3 and a bypass stream 19. The bypass stream was returned to the reactorfluidized bed 2, at an elevation above that of the polymer productdischarge streams 8 and 8′. The separation was conducted such that theliquid content of the bypass stream was enriched and the liquid contentof the primary recycle stream was reduced. The test cases with reducedliquid at the reactor inlet, resulted with reduced liquid in the fluidmixture exiting with the solid polymer product in the discharge streams8 and 8′.

Example 17 includes a more detailed discussion of the discharge processsteps, the results of which are summarized in Table 8 for this Example19.

For the four test cases in this Example 19 the polymer product had asolid phase density of 57.3 lb/ft{circumflex over ( )}3. For a givensemi-batch discharge cycle, the solid polymer product occupied a volumeof 20.02 ft{circumflex over ( )}3 in the vessels. Each discharge eventremoved 1,147 lb of solid polymer product from the reactor fluidized bed2. The balance of volume was occupied by raw material fluids. Thesefluids were in the vapor and optionally liquid phase. They comprisedunreacted monomers, comonomers and inert species. The total volume inthe interconnecting system for vessels 10 and 10′ was 102% of theavailable volume for vessels 12 and 12′.

The total time duration of the discharge process is illustrated in Table8. Cases with multiple venting and equalization steps are included. Timeduration for valves to open and close are included in each event. Thetotal time duration includes several sequential batch steps. These wereidentified in Example 17.

To complete the discharge process, the solid polymer product and someremaining raw material fluids was transferred through either lines 14 or14′, to downstream processing equipment. The first vessel equipment intowhich vent was taken and transfer was completed had an available volumeof 6 times the available volume in vessel 10.

The product removal capacity of the production facility was determinedby the time necessary to economically complete a discharge and pressureequalization cycle. When operating in “condensed mode”, with thedischarge of fluid containing some liquid, the time duration necessaryto complete the intermittent discharge process was shown to increase asthe quantity of liquid in the discharge streams 8 and 8′ was increased.

In this Example 19 the product removal capacity of the discharge systemdecreased from 25,727 to 25,663 lb polymer per hour as the liquidconcentration in streams 8 or 8′ increased from 4.72 to 20.01 weightpercent. Conversely, lower concentrations in streams 8 and 8′ resultedin increased product removal capacity even in the condensed mode.

Also included in Table 8 is a summary of the quantity of raw materialmass transferred during the pressure equalization steps. Raw materialmass, which was not transferred to a parallel vessel by equalization,leaves the discharge process with the solid polymer product. Foreconomical operation a portion of these fluids must be processed forrecovery to the reaction system. The raw material efficiency wasaffected by the minimum pressure obtained by the steps of vesselpressure equalization. At increasing final pressure, a larger quantityof the valuable raw materials would exits with the solid polymer productin streams 14 or 14′. These fluid materials are lost or must beprocessed by downstream operations for recovery.

The “removal ratio” decreased with decreasing liquid quantity in thedischarge. The cost of recovering or loosing valuable raw materialincreases with liquid quantity in the discharge, which in turn isgoverned by the quantity of liquid injected through line 19 rather thanline 3. In this Example 19 the “removal ratio” increased from 0.0195 to0.0216 lb fluid per lb of polymer as the as the liquid concentration instreams 8 or 8′ increased from 4.72 to 20.01 weight percent. TABLE 8POLYETHYLENE BUTENE COPOLYMER DISCHARGE CYCLE COMPARISON OF LIQUIDCONTENT IN DISCHARGE STREAMS 8 AND 8′ Case 1 2 3 4 Stream 67 CoolerOutlet Liquid 17.34 17.34 17.34 17.34 (weight %) Stream 8 DischargeFluid Excluding Solids Vapor Density 2.10 2.12 2.12 2.19(lb/ft{circumflex over ( )}3) Liquid Density 34.27 34.09 33.74 33.45(lb/ft{circumflex over ( )}3) Liquid 20.01 17.14 10.76 4.72 (weight %)Profile Vessel Pressure (psia) 10 After 364.7 364.7 364.7 364.7Discharge 10 Equalization 263.7 260.7 259.5 253.8 with 10′ 12, 12′Before 20.0 20.0 20.0 20.0 Transfer 10 After 176.0 172.6 171.0 165.0Transfer to 12 12 Equalization 103.1 100.9 100.3 95.8 with 12′ 12 AfterFinal 20.0 20.0 20.0 20.0 Transfer Pressure Equalization Recovered Mass(lb) 10 Equalize 38.5 38.6 39.2 40.5 with 10′ 10 Transfer 32.0 31.3 31.531.2 to 12 12 Equalize 24.7 24.0 23.8 22.5 with 12′ Stream 14 TransferSolid with Fluid to Downstream Processes Solid Product 1,147. 1,147.1,147. 1,147. (lb) Fluid with 24.8 24.0 23.8 22.4 Solid (lb) Stream 14Removal Ratio Fluid/Solid 0.0216 0.0209 0.0207 0.0195 Profile TimeDuration (seconds) 10 Discharge 22.5 22.5 22.4 22.4 and Vent 10 Equalize26.1 26.0 26.1 26.5 with 10′ 10 Transfer 36.1 36.1 36.1 36.1 to 12 12Equalize 14.9 14.8 14.8 14.4 with 12′ 12 Final 61.3 61.2 61.2 61.1Transfer Total Cycle 160.9 160.6 160.6 160.5 Duration Product 25,663.25,711. 25,711. 25,727. Discharge (lb/br)

1. Fluid recycle apparatus for a fluidized bed polyolefin reactorcomprising a conduit for removing recycle fluid from said reactor,compressor means in said conduit for compressing fluid therein, coolermeans in said conduit for cooling fluid therein, and a splitter having aprimary outlet and at least one slip stream conduit, for receivingpartially condensed recycle fluid from said cooler means and returningit by said primary outlet portion to a level below said fluidized bedand by said slip stream conduit through a direct passage to a reactionzone within said fluidized bed.
 2. Fluid recycle apparatus of claim 15wherein said slip stream conduit has a hydraulic diameter of 5% to 30%of the diameter of said primary outlet.